What It Is:
: a heavy catapult used in ancient and medieval times
About the Word:
Onager has an ancestor in the Greek onos, meaning "ass." That term developed into both the four-legged onager (an Asian wild ass) and the wooden onager (the catapult).
Like its animal namesake, the catapult onager had a tendency to kick up its rear end – though the catapult only did this when its stone was discharged.